<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<p id="demo"></p>

<script>
    var s = '', n, t = null;
//    alert("s: " + s + " n: " + n + " t: " + t);
//    if (s === "") alert("s");
//    if (n == null) alert("n");
    if (t == null) alert("t");
    document.getElementById("demo").innerHTML = "Paragraph changed.";
</script>

</body>
</html>